doc: remove soc.txt

The file was written in 2006, it was never updated nor really used as
reference for SOC-like programs.
Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Stefano Sabatini <stefasab@gmail.com>

doc/soc.txt

-Google Summer of Code and similar project guidelines
-
-Summer of Code is a project by Google in which students are paid to implement
-some nice new features for various participating open source projects ...
-
-This text is a collection of things to take care of for the next soc as
-it's a little late for this year's soc (2006).
-
-The Goal:
-Our goal in respect to soc is and must be of course exactly one thing and
-that is to improve FFmpeg, to reach this goal, code must
-* conform to the development policy and patch submission guidelines
-* must improve FFmpeg somehow (faster, smaller, "better",
-  more codecs supported, fewer bugs, cleaner, ...)
-
-for mentors and other developers to help students to reach that goal it is
-essential that changes to their codebase are publicly visible, clean and
-easy reviewable that again leads us to:
-* use of a revision control system like git
-* separation of cosmetic from non-cosmetic changes (this is almost entirely
-  ignored by mentors and students in soc 2006 which might lead to a surprise
-  when the code will be reviewed at the end before a possible inclusion in
-  FFmpeg, individual changes were generally not reviewable due to cosmetics).
-* frequent commits, so that comments can be provided early